Cathode ray tube



' March 19, 1940..

L. F. BRoAnwAY Er AL 2,194,380

CATHODE RAY TUBE Filed March 21, 1935 2 Sheets-SheetI l March 19, 1940. L} F. BROADWAY Er A1. 2,194,380

CATHODE RAY TUBE Filed March 21, 19:55 2 sheets-sneer 2 5ba/ffm /oorfwr/a Y Patented Max'. 19, 1940 parar CATHODE RAY TUBE i (1f Application March 21,

1935, Serial No, 12,214

In Great Britain March 26, 1934 Claims.

The present invention relates to improvements in circuit arrangements comprising cathode ray tubes.

Cathode ray tubes for television and like purt poses generally comprise a cathode, a modulating electrode and one or more anodes arranged within an evacuated envelope; means for deflecting the ray over va liuorescent screen associated with the tube may also be provided. The anode or anodes are maintained, in operation, at positive potentials relative to the cathode; these electrodes cooperate with one another in accelerating electrons from the cathode and focusing them upon the fluorescent screen.

It is an object of the present invention to provide new and improved circuit arrangements incorporating cathode ray tubes.

According to the present invention, a circuit arrangement comprises a cathode ray tube of the '20 kind in which means, such as one or more modulating electrodes, are provided for modulating the intensity of the ray in accordance with an applied modulating potential associated with means for varying the mean slope oi the curve representing the relation between the modulating potential and the fluorescent screen current of the tube without altering the disposition of the electrodes thereof.

The mean slope of the modulating potentialfluorescent screen current characteristic may be dened as the ratio oi the change in screen current to the corresponding change in modulating potential, and .it will be seen that unless the characteristic is rectilinear, the value of the mean slope will depend upon the limiting values of the change in modulating potential considered. Thus when in this specification a change in mean slope is mentioned, it will be assumed that the change is measured for the same range of modulating L0 potential.- i

According to a feature of the invention, a circuit arrangement comprises acathode ray tube of the kind in which i'neansl such as one or more modulating electrodes, are provided for modu- 4? lating the intensity of the ray in accordance with an applied modulating potential associated with means for Varying the curvature of thecurve representing the relation between the modulating potential and the fluorescent screen current "o of the tube without altering the disposition of the electrodes thereof.

As will appear hereinafter, changes in mean slope of the `characteristic may be accompanied by changes in curvature thereof and vice versa; it must be borne in mind, however, that when a,

lil

(Cl. 25ll--27) considerable changes in mean slope may be accompanied by only slight changes in curvature. Further reference is made to this point below. l

A cathode ray tube may comprise, in addition l to the electrodes mentioned above, a further electrode (which will hereinafter be referred to as.v`

the accelerating electrode or accelerator) arranged between the cathode and the modulating electrode. A cathode shield or screening electrcde is also usually provided lclose to the cathode. near cathode potential and electrons pass outwards throughan aperture therein. In such a tube, the -accelerator is maintained at a positive potential relative to the cathode, while the modulator is given a negative bias, and the arrangement is such that electrons from the cathode are rst accelerated by the accelerating electrode, then decelerated by the modulator, and then again accelerated by the anodes. When the potentials applied to the electrodes are suitably, chosen, it is found that as the potential'applied to the modulator is made more negative'the electron current flowing directly to the accelera- This electrode may be maintained at ont,

tor electrode increases, while the current flowing f to the fluorescent screen decreases. can be modulated in the usual manner in accordance with signals applied between the modulator and the cathode.

The tube described is an example of one in which electrons from a cathode are rst accel. erated and then decelerated, before being finally accelerated and focused upon the screen, and in the present specification, such tubes will be referred to for convenience as being of the double acceleration type. All tubes of this type will be regarded as characterised in that an accelerating electrode is disposed between the cathode and the modulator electrode. It is to be understood that in a tube of the double acceleration type, the electrons may be subjected to more than two accelera-tions during their passage from the cathode to the screen; furthermore, it must be appreciated that, between the two accelerations in question, the electrons are subjected to a deceleration. v

rlubes oi the double acceleration type form the subject of co-pending application Serial No. 745,- 838. The tubes described in the specification of the above-numbered application are examples of tubes which are suitable for use in circuit ar-` The beam f rangements according to the present invention.

According to a further feature of the invention, the tube employed is of the double acceleration type and variations in mean slope are effected by varying the value of a resistance inserted between the accelerating electrode of the tube and the associated source of potential dif` ference; for effecting variations in the curvature of 'the characteristic, means are provided. for increasing the effective potential difference furnished by this source as the value of this resistance is increased.

According to another aspect of the invention, a circuit arrangement comprises a cathode ray tube of the double acceleration type having a cathode, an accelerating electrode, a rmodulator electrode, one or more anodes, and a iiuorescent screen and means associated with the tube for supplying operating potentials for the electrodes thereof, a resistance being arranged in the connection between the accelerating electrode and 'the said means,

The invention will be described by way of example with reference to the accompanying diagrammatic drawings in which Fig. 1 illustrates one method of carrying the invention into eiect, and shows a cathode 'ray tube and part of the associated circuit thereof,

Fig. 2 illustrates a modification of the invention, and

Fig. 3 is an explanatory figure.

Like parts in Figures 1 and 2 are given the same references.

Referring to Fig. l, a cathode ray tube comprises an evacuated, sealed glass envelope I consisting of a cylindrical portion flaring out into a frusto-conical portion. Disposed within the envelope I, and mounted therein in any known or suitable manner, are a cathode 2 of the indirectly heated type, a cathode shield 3, an accelerator electrode 4, a modulator electrode or grid 5, a rst anode E and a second anode 1. The electrodes 3, 4, 5 and 6 are all of cylindrical shape and may be mounted in any convenient manner, for example, as sliding nts within a tubular member of glass (not shown) arranged within the cylindrical portion of the envelope I, while the electrode l may be mounted upon the wall of the envelope as shown, for example in the form of a silvering of the surface of the envelope; the cathode may be supported upon its own lead and the leads to its heater.

Although in the drawings the leads to the various electrodes are taken out through the side wall of the envelope, it is to be understood that` in practice some or all of these leads may be taken out through a foot t sealed into the end of the cylindrical portion of the envelope.

The second anode 'l is arranged slightly to overlap the rst anode 6, these two electrodes constituting an electron lens and serving to focus a beam of electrons upon a uorescent screen 9 disposed on the end wall of the frusto-conical portion of the envelope l. Either electrostatic or electromagnetic deflecting means (not shown) are provided for cleecting the beam and for causing it to scan the fluorescent screen.

The tube described is substantially the same as that described with reference to Fig. 1 of the drawings accompanying the specication of the above-mentioned application, and may have substantially the same dimensions as the tube there described.

Referring again to Fig. l of the accompanying drawings, the various electrodes of the tube all derive their operating potentials from a potential divider I0, Il which is fed with direct current from a rectifier I2 fed with alternating current from the mains throughl a step-up transformer I3. It is arranged that the potential at the terminal I0 of the potential divider, and hence the potential on the second anode l, relative to earth is about 3,600 volts.

The first anode 6 is connected to a tapping point I4 in the potential divider, and this tapping point may be made adjustable so as to provide for adjustment of the focus of the beam; the first anode potential may conveniently be made about 1000 volts. The grid 5 is connected to earth through a suitable input circuit which is connected between the terminals I5; the input circuit may, for example, comprise a source of picture signals such as a wireless receiver.

The accelerator Il is connected through a variable resistance I5 to an adjustable tapping point Il in the potential divider, such that a potential of the order of 250 volts relative to earth is applied to the accelerator. The cathode 2 and the cathode screen 3 are connected together and to a point IB in the potential divider such that a potential of about 20 volts positive relative to earth is applied to these electrodes. Decoupling condensers I9, 20 and 2l are vconnected between the tapping points I4, Il and I8 respectively and earth.

With such an arrangement of electrodes, and with such a choice of the potentials applied thereto, electrons from the cathode are accelerated by accelerator 4, and decelerated by the grid 5. Electrons passing through the aperture in the grid 5 are then acceleratedby electrodes 6 and 'l and focused thereby upon the screen 9.

Now in a tube such as that described, if the potentials (relative to the cathode) on the cathode screen, the grid and the rst and second anodes L are kept constant at about O, 20, 1000 and 3300 respectively and the fluorescent screen current is measured for different values of accelerator voltage, it will be found that the fluorescent screen cunrent increases approximately parabolically with increasing values of accelerator voltage. Thus the fluorescent screen current may increase from zero at Zero accelerator voltage at an increasing rate.

If the accelerator voltage is kept constant, for example at about 250 volts relatively to the cathode, and the grid voltage is increased from Zero in a negative sense relatively to the cathode, it will be found that the accelerator current increases substantially rectilinearly whilst the fluorescent screen current and the first anode current decrease substantially rectilinearly and at approximately the same rate. The curve A in Fig. 3 shows the rectilinear relationship between the grid (or modulator) potential and the uorescent lscreen current; in Fig. 3, the modulatorcathode potential difference is plotted against fluorescent screen currents as ordinates.

The value of the accelerator current for zero grid voltage may be taken to represent thecurrent which ilows directly from the cathode to the accelerating electrode. The rectilinear increase vof the'acceleratorcurrent with increasing negative grid. voltage may be interpreted as being due to the electrons which are returned to the accelerating electrode by the repulsive field of the grid.

If, however, according to one aspect of this invention, a resistance of value R be arranged in series between the accelerating electrode and the source of current from which it is supplied, that is, referring to the drawings, between the electrode il and the tapping point Il, the voltage EA at the accelerating electrode will be given by where V is the voltage of the source and IA the current through the resistance R.

ince IA increases with increasing negative grid voltage. it followsthat EA decreases and therefore the total current drawn from the cathode by the accelerator voltage is decreased. This has the effect of making the mean slope of the fluorescent screen current-grid voltage characteristic curve of the tube greater than Where the resistance R is omitted and the accelerator voltage is kept constant. Hence if the value R be made adjustable, the mean slope of the grid volts-fluorescent screen current characteristic can be controlled without altering the dispositions of the electrodes of the tube. In the arrangement described, the resistance i6 is made variable so as to provide a control of the mean slope of the characteristic in the manner set forth. It is to be noted that the control thus obtained has substantially no adverse effect on the focusing of the beam.

The tube described may have a substantially rectilinear grid volts-uorescent screen current characteristic curve as already explained; this may be of advantage for such purposes as television reception because modulating potentials of substantially all amplitudes applied between the cathode and grid are converted into light Values substantially Without any selective effect. If in addititon, control of the mean slope of the linear characteristic is provided, the modulation sensitivity of the tube may be controlled thus giving the equivalent of volume control (with no frequency selection) in audio receivers.

It is also found that where, in the case of tubes having no resistance inserted in the lead to the accelerator, the grid volts-fluorescent screen current characteristic is not quite rectilinear but is slightly concave, as seen from the origin, the insertion of such a resistance of suitable value may have the eiect of straightening the characteristc, at least to a small extent. Further increase in the value of the resistance may cause the characteristic to depart from rectilinearity and to tend to become convex, as seen from the origin. Control of the resistance may therefore also be used in some cases to control the cunrvature of the characteristic. This is of advantage where it is desired to give the tube a characteristic curve such as will compensate for a nonlinear response of another member of the apparatus associated with the tube, as for example, a thermionic valve amplifying the signals .to be applied to the tube and working on a curved portion of its characteristic.

In the case of nlm television control of the curvature of the tube characteristic may also be used to compensate for an unsuitable value of the relative contrast in the dark and light portions of the nlm being transmitted. It is at present common practice in the nlm industry to develop motion picture iilrns in such manner that detail is brought out in the light portions of the picture relative to the detail in the dark portions, lms developed in this way appearing quite natural to eye when projected upon a screen. A convenient value of intensity contrast for the developed film may be about 1.8, for example. This means that if D is the density of the lm, then I =lcD18 where I is the intensity of the light from the subject and 1c is a constant. If, however, the lm is developed to an intensity contrast of less than, say, 1.8, this lack of intensity contrast may be compensated, at least to some extent, by Varying the resistance in the accelerator lead as described above.

A cathode ray tube having a rectilinear characteristic may be regarded as having an intensity contrast of unity, while if the characteristic is parabolic, the intensity contrast is 2, and so on. Increasing the curvature of the characteristic so as to make it more convex, as seen from the origin, may thus loe regarded as increasing the intensity contrast of the tube.

It is found that a more satisfactory control of the intensity contrast of a tube may be obtained if it is arranged that as the accelerator circuit resistance is increased, the e'ective value of the source of accelerator potential is also increased. In the arrangement described, this may be achieved by arranging that as resistance EE is increased, the position of the tapping point I1 is moved to the right, that is, towards the higherpotential end of the potential divider. Preferably it is arranged that adjustment of the value of resistance lli and of the position of tapping point Il may be achieved'by a single control. A suitable maximum value for the resistance IB. is about 250,000 ohms. It is preferably arranged that the resistance of the part of the potentiometer l0, il between the tapping points l'! and I8 is small compared with the operating values of the resistance E6, so that changes in the position of tapping point i1 introduce negligible changes in the total resistance in the accelerator circuit.

It is to be noted that changes in the mean slope of the characteristic vof the tube, and changes in the intensity contrast of the tube, cannot in general be effected separately, a change in one being accompanied by a change in the other. It is generally found that an increase in intensity contrast .obtained as above described will produce an increase in the mean slope of that part of the characteristic corresponding to the larger ray intensities but the' same increase in intensity contrast might produce a decrease in mean slope for a restricted range of grid potentials close to the grid potential corresponding to the lower ray intensities. In practice, an increase in intensity contrast is usually accompanied by an increase in the mean slope of the characteristic for the range of grid potentials utilised.

It is to be noted that the diminution of accelerator voltage with increasing negative grid volts operates to protect the cathode of the tube since the latter has a shorter life and is less stable when too large a current is drawn through the tube.

The resistance in the accelerator lead also operates as a safety circuit, to some degree, if there occurs an insulation break-down within the tube.

An alternative method of controlling the curvature of the characteristic curve representing the relation between the modulating potential and the fluorescent screen current of the tube, that is, of controlling the intensity contrast of the tube, will now be described with reference to Fig. 2.

It has already been stated that if the cathode screen, the accelerator and the two anodes of a cathode ray tube of the double acceleration type lare held at constant'potentials, a substantiallyrectilinear relationship (curve A in 3) between modulator electrode potential and fluorescent screen current is obtained. It is also found that if the potentials of the accelerator, the modulator and the two anodes are held at suitable constant values', the relationship bet veen cathode screen potential and uorescent screen current such that the curve representing this .relationship is convex as seen from the origin that is to say tlie'intensity contrast is than unity: this curve may have the forni. of the curve B in Fig. 3.

Il the potentials of the modulator and the cabeo.` lreen are caused to vary sinq tancously in depen ice upon the same modulaing potential, the relationship between this modulating potential and the fluorescent scr-:een current yields curve, such the curve C of Fig. 3. is inteimediate in curvature between the substantially rectilinear modulator characteristic -nd the curved cathode screen character .tic e amount of the curvature of curve C being dependent upon the ratio of the modulating potentials applied to the modulator and to the cathode screen. The method of controlling intensity contrast which is illustrated in 2 is based upon the above facts.

Modulating pete: Vals such as picture signals for example are derived 'from a tlierinionic valve E2 lie-ving an anode resistance the va ve 22 may Vform part oi a wireless receiver. The anode of the valve 22 is connected through a condenser 2li a res'otance in series to an adjustable tapping pouit in a part 2l of the potential divider' i3. il. n adjustable tapping point 28 in resistance 25 is connected to the cathode shield 3 o the tube. The anode of valve 22 is also connected through condenser and resistance 3G in series to a tapping point 3i in a potential divider 3:2 connected in shunt ith the part El'. An adjustable tapping point 33 in resistance Sil is connected to the modulator E of the tube.

By he positions of tapping points 2S and either separately or simultaneously by mear of a ganged control,- the ratio of the modulating potential applied to the cathode screen to that applied to the modulator may be varied, adjustment of t e intensity contrast introduced by he tube be obtained.

Increasing the magnitude of the modulating potential applied to the modulator, while simultaneously decreasing the module. g potential applied to the cathode screen, as above described, produces a decrease in the mean value of the fluorescent screen current, and vice Versa. Such changes may be compensated by Varying the biasing potentials applied to he modulator and to the cathode screen by adjusting the position of either or both of the tapping points 45 and 3l. The arrangement may be rnade such that as the modulating potential applied tc the modulator is uw., and that applied to the cathode screen is decreased, the biasing potential applied to the tor is made more negative, while that appl to the cathode screen is made less negative, and vice versa.

adju. ynient of the positions of anyv two or more oi' the tapping points 3.6, Si and (i3 may be effected multauecusly by means of a single control member.

We claim:

l. A circuit arrangement comprising a cathode ray tube having a cathode for emitting a beam of electrons, a screen for receiving said beam and, arranged between said cathode and said screen in the order named, an accelerator electrode, a modulator electrode and at least one anode, means for biasing said modulator electrode to a negative potential relative to said cathode, means for applying modulating potentials to said modulator electrode, a Source of current having a negative terminal and a tapping point adjustable to any of a number of positions at positive potentials relative to said negative terminal, a con* nection between said negative terminal and said cathode, and a connection including a variable resistance between said tapping point and said accelerator electrode for varying the potential supplied to the accelerator electrode so as to vary the curvature of the curve relating said modulating potentials and the number of electrons received at the screen.

2. A circuit arrangement comprising cathode ray tube having a cathode for emitting a beam of electrons, a screen for receiving said beam and, arranged between said cathode and said screen in the order named, a screen electrode, an accelerator electrode, a modulator electrode and at least one anode, means for maintaining said second electrode and said modulator electrode at negative potentiak relative to said cathode, means for maintaining said accelerator electrode at a positive potential relative to said cathode, means for applying modulating potentials to said modulator electrode and to said screen electrode and means for varying the ratio of the modulating potential applied to said modulator' to the modulating potential applied to said screen electrode.

3. A circuit arrangement comprising a cathode ray tube having a cathode for emitting a beam of electrons, a fluorescent screen for receiving said beam and, arranged between said cathode and said screen in the order named, a scree electrode, an accelerator electrode, a modulator electrode and at least one anode, means for applying modulating poten ls to said modulator electrode and to said screen electrode, means for applying to said modulator and to said screen electrode bias potentials relative to said cathode, means lor varying the ratio of the modulating potential applied to said modulator to the modulating potential applied to said second electrode and means for varying the magnitudes of said bias potentials.

4;. A circuit arrangement comprising :athode ray tube having a cathode for eniitt' a beam of electrons, a fluorescent screen for receiving said beam and, arranged between cathode and said screen in the order a screen` electrode, an accelerator electrode, a modulator electrode and at least one anode, means for applying modulating potentials to said modulator electrode and to said screen electrode, means for applying to said modulator and to screen electrode bias potentials relative to said cathode and a control member for varying simultaneously the ratio of the modulating otential applied to said modulator to the modulating potential i plied to said screen electrode and the mai, tudes of said bias potentials.

5. A circuit arrangement compri ray tube having a cathode for emi of electrons, a screen for recev arranged between said cathode and said so: the order named, a shield electrode, au accelerator electrode, a. modulator electrode, and `at l one anode, means for biasing said s d electrode to a negative potential relative to said cathode,

1"ig a c inode ig a beam said lucani and,

negative potential relative to said cathode, means for biasing said accelerator electrode to a positive potential relative to said cathode, means for applying modulating potentials to said modulating electrode', and meansfor varying the accelerator electrode potential so as to vary the curvature of the curve relating said modulating potentials and the number of electrons received at the screen.

6. A circuit arrangement comprising a cathode ray tube having a cathode for emitting a beam of electrons, a screen for receiving said beam and, arranged between said cathode and said screen in the order named, a shield electrode, an accelera* tor electrode, a modulator electrode, and at least one anode, means for biasing said shield electrode to a negative potential relative to said cathode, means for biasing said modulator electrode to a negative potential relative to said cathode, means for biasing said accelerator electrode to a positive potential relative to said cathode, means for applying modulating potentials to said modulating electrode, and variable resistance means for varying the accelerator electrode potential so as to vary the curvature of the curve relating said modulating potentials and the number of electrons received at the screen.

7. A circuit arrangement comprising a cathode ray tube having a cathode for emitting a beam of electrons, a screen for receiving said beam and, arranged between said cathode and said screen in the order named, an accelerator electrode, a modulator electrode and at least one anode, means for biasing said modulator electrode to a negative potential relative to said cathode, means for biasing said accelerator electrode to a positive potential relative to said cathode, means for applying modulating potentials to said modulator electrode, means for varying the accelerator electrode potential so as to vary the curvature of the curve relating said modulating potentials and the number of electrons received at the screen and means for varying the biasing potential applied to said accelerator electrode.

8. A circuit arrangement comprising a cathode ray tube having a cathode for emitting a beam of electrons, a screen for receiving said beam and, arranged between said cathode and said screen in the order named, an accelerator electrode, a modulator electrode and at least one anode, means for biasing said modulator electrode to a negative potential relative to said cathode, means for biasing said accelerator electrode to a positive potential relative to said cathode, means for apl plying modulating potentials to said modulator electrode, means for varying the accelerator electrode potential so as to vary the curvature of the curve relating said modulating potentials and the number of electrons received at the screen and potentiometer means for varying the biasing potential applied to said accelerator electrode.

9. 'I'he method of controlling the modulation sensitivity of a cathode ray tube wherein is provided a source of electrons comprising the steps of providing a preliminary acceleration field of substantially constant order for said electrons from said source, developing signalling energy, modulating the electrons in the region intermediate the source and said preliminary acceleration field by the developed signalling energy, varying the acceleration subsequent to the preliminary acceleration of said electrons under the control of the developed signalling energy,

and subjecting said subsequently accelerated elec-y trons to a further constant acceleration ield.

li). The method of controlling the modulation sensitivity of a cathode ray tube wherein is provided a source of electrons comprising the steps of providing a preliminary acceleration eld of substantially constant order for said electrons from said source, developing signalling energy, modulating the electrons in the region intermediate the source and said preliminary acceleration eld by the developed signalling energy, subsequently reducing the acceleration of said modulated electrons by an amount proportional to the magnitude of said developed signalling energy, and subjecting said decelerated electrons to a constant accelerating fieldl LEONARD FRANCIS BROADWAY. WILLIAM FRANCIS TEDl-IAM. 

